1. Field of the Invention
The present invention relates generally to uninterruptible power supply (UPS) systems, and more particularly to two or more UPS systems connected and operated in parallel wherein pulse width modulated (PWM) pulse synchronization between the UPS systems takes place using the locally measured critical bus voltage to detect zero crossing.
2. Background of the Invention
A UPS system is used in applications that require continuity of power, such that when the main power source fails, the UPS system provides power from a reserve energy stored system, typically in the form of a battery. The UPS system monitors the main power source and controls the UPS components to provide continuous power to a critical load.
To provide further continuity of power, multiple UPS units may be paralleled with other UPS units to form a redundant and scalable UPS system with a given multiplicity. This system adds redundancy in that any UPS unit may be disconnected from the critical load for service or automatically by the UPS unit to isolate a faulty unit. The remaining unit(s) still provide power to the critical load. In addition, this system allows flexibility in increasing the total power capacity of the system. Typically, when UPS units are paralleled, some unit-to-unit wiring is added to facilitate the sharing of power and the connection or disconnection of units.
In parallel UPS systems, the PWM pulse switching periods are required to be synchronized to minimize circulating currents. Due to a fundamental limitation of digital control law, a digital load sharing control loop can only affect harmonic components of the output currents from the fundamental frequency up to one-half of the PWM pulse frequency. Even when proper load sharing control at relatively low frequency can be achieved using an appropriate digital load sharing loop, there are instances when high frequency components of the circulating currents exist. These high frequency circulating currents arise as a result of PWM pulses not being synchronized when multiple UPS units are connected in parallel.
In order to minimize these high frequency circulating currents, the PWM pulses must be synchronized in all units of the parallel connected UPS system. Typically, synchronization is accomplished using a “master” controller which sends a synchronization pulse to all other units in the UPS system where the other units act as “slaves.” All the “slave” units will then synchronize their PWM pulses to the “master's” PWM pulses. Although this technique is acceptable, this technique requires additional wiring between units to transmit the synchronization pulse, and the existence of “master-slave” relationships results in single point failure which decreases the reliability of the overall system.
A need has thus arisen for a method for PWM pulse synchronization in a parallel UPS system which does not require additional wiring between units and does not rely on a “master-slave” controller relationship.